How do I replace a rotten shed floor?

Getting the old deck out, dealing with the bearers and the base underneath, and building the new floor so it does not rot again.

Difficulty: intermediate
Time: 5 hr
Read: 4 min
Safety: caution

Short answer

Empty the shed, lift the old boards, and check the bearers — they are usually the real problem. Replace any soft bearers with treated timber on a membrane or on pads that lift them clear of the ground, then lay new treated ply or boards.

Shed floors rot from underneath, which is why they look fine until the day someone puts a foot through one. The rot is almost always caused by the bearers sitting on damp ground with no air beneath them, so replacing only the boards buys a couple of years at most.

Safety

A rotten shed floor can give way underfoot without warning, especially around the door where the wear is. Test with a foot on a plank before putting weight on it. Empty the shed before working on the floor, and prop the walls if the floor is structural to the frame.

What you'll need

18mm exterior or treated plywood, or treated floorboards
Treated timber bearers, 75mm x 47mm or heavier
Damp-proof membrane or bearer pads
Exterior screws
A crowbar and a circular saw or handsaw

Step by step

Empty the shed completely and check the walls.In many sheds the walls sit on and are fixed to the floor. If so, you will need to prop the walls or work in sections rather than removing the whole floor at once.
Lift the old deck and inspect the bearers.Push a screwdriver into each bearer along its length. Soft timber, especially where it touches the ground, means the bearer goes too. This is usually the case.
Find out why it rotted.Ground contact, standing water under the shed, blocked airflow, or a leak. Replacing the floor without fixing the cause guarantees a repeat.
Improve the base before rebuilding.Level the ground, add a permeable membrane and gravel, or set concrete or plastic bearer pads. The bearers must sit clear of the soil with air able to move under them.
Set the new bearers at the right spacing.400mm centres for 18mm ply, closer if the shed will carry heavy items. Level them across the whole footprint with a long straightedge and a level.
Treat every cut end.Sawn ends of treated timber are untreated. End-grain preserver on every cut, and a second coat on any face that will be hard to reach later.
Lay the deck with the joints on bearers.Plywood sheet edges must land on a bearer. Add noggins where a joint falls between them. Screw at 300mm centres along every bearer.
Leave a small gap at the walls.2-3mm around the perimeter lets the deck expand without lifting, and stops water sitting in the joint. Seal the perimeter with a flexible sealant if the walls sit on the floor.
Seal or paint the floor before loading it.A floor paint or an exterior sealer, including the underside of any board you can still reach, is what keeps the new floor going.

Tips

Do the floor and the base at the same time. Access to the base is never going to be easier than with the floor out.
Treated plywood outlasts untreated exterior grade by a wide margin in a shed and costs very little more.
Photograph how the wall panels meet the floor before you take anything apart.

Common mistakes

Replacing the boards but reusing the bearers — The bearers are the part in contact with the damp ground and are usually the source of the rot. New boards on rotten bearers fail within a couple of years.
Laying the new floor straight onto soil or slabs with no air gap — Timber in permanent contact with a damp surface never dries. The bearers must be on pads or a membrane with airflow beneath.

If it doesn't work

Floor springy in the middle of a bay

Cause: Bearer spacing too wide for the deck thickness — Fix: Add an extra bearer between the existing ones, or use thicker deck material.

New floor damp within months

Cause: Standing water under the shed — Fix: Regrade the ground so it falls away, add a gravel layer, and check gutters and downpipes are not discharging beside the shed.

Wall panels sagging once the floor came out

Cause: Walls were relying on the floor for support — Fix: Prop before removing the floor, or work in half-widths so half the floor is always in place.

Boards cupping upwards

Cause: Wetter underneath than on top — Fix: Improve underfloor ventilation and seal both faces of the boards, not just the visible one.

Questions people ask

Can I just screw a new floor over the old one?

Only if the bearers are sound and the rot is confined to the surface, which is rare. Overlaying traps the rotten timber against the new deck and moves the floor level up past the door threshold.

What is the best material for a shed floor?

Treated 18mm plywood on treated bearers, sealed on both faces. It is stronger per millimetre than boards, has no gaps for things to fall through, and takes floor paint well.